Hello conrad13,
Those are good questions to be asking based on your situation with a low data plan. The best way to know the differences between the two is that if you are sending an iMessage, then you will see a blue text bubble when the message is sent. If it is using a standard text message, it will be a green bubble. If you have friends that have an iPhone and registered for iMessage, then it will send iMessages over the standard text message. If you want to make sure that you send text messages then you will need to turn iMessage of in Settings > Messages.
To make sure that when you compose a message that is being sent from your phone number, you will need to check the settings by going to Settings > Messages > Send & Receive and scroll down until you see Start New Conversation From. Keep in mind that if people send you a message that is a different address and reply, then you will be reply from that address and not the one you necessary want to start from. For more details, take a look at the articles below.
Send messages with your iPhone, iPad, or iPod touch
https://support.apple.com/en-us/HT201287
iOS and OS X: Link your phone number and Apple ID for use with FaceTime and iMessage
https://support.apple.com/en-us/HT201349
Regards,
-Norm G.